skin discoloration

hey doc,
my 5 yr old pug has had it seems a sudden skin discoloration on the non hair parts of her body, it has gone from a rosy color to a dirty color brown which resembles dirt but does not wash off, she is otherwise very health and is up to date on her vet visits and shots.what do you think?she is spayed if that helps
   monkey 98

[Aleda M Cheng, D.V.M., C.V.A]

The most common cause of skin discoloration is atopy (skin allergies), and is due to the accumulation of Malassezia skin yeast, or bacteria on the skin.  Endocrine disorders such as hypothyroid or hyperadrenocorticism (Cushing's syndrome) can also cause skin discoloration.

Two less common causes are:  Lentigo, which is an inherited hyperpigmentation in Pugs, and your dog is half pug.   It causes circular brownish-black skin lesions on hairless areas; and, Demodex mites.